This continuation application proposes to extensively address two general questions about STI during the developmental transition from middle and late adolescence into young adulthood: 1) Describe the developmental epidemiology of four sexually transmitted organisms (Chlamydia trachomatis, Neisseria gonorrhoeae, Trichomonas vaginalis, and Mycoplasma genitalium) among adolescent and young adult women and among their male sex partners; 2) examine patterns of infection acquisition, duration and resolution using weekly vaginal swabs over 12-week intensive observation periods. Data from the first question may inform relevant clinical and policy issues such as the initiation, frequency and duration of STI screening programs. Data from the second question could additionally inform issues related to prevention interventions and partner treatment, among others. Data will be collected from re-enrolled participants in current project and are between 14 - 21 years of age. Quarterly visits (over four years) and two 3- month intervals of daily diary and weekly vaginal swab collection will be merged with data collected during the current project period. New data allow focus on STI related issue in late adolescence and early young adulthood. Merged data from both projects (up to 75 months of follow-up) allows focus on a full developmental span from middle adolescence into young adulthood.